Moderate Interference

A moderate interference site sits between the clean openness of a low environment and the demanding complexity of high interference. These are the most common sites you will encounter in a busy Australian metro market — shopping centre surveys, station precincts, medium-density residential. The defining characteristic is partial sky obstruction: clear overhead, but taller structures at the edge of your view cut off the lower horizon.

Key Rule

GPS works but can drift in the afternoon as satellite geometry degrades. The failure mode here is overconfidence — everything looks fine at power-on, but the data tells a different story by 12:00. Watch the position indicator before arming. It tells you something real at this site type.

Moderate interference is created by the transition zone between open suburban land and denser urban development. Low-angle satellites — those near the horizon — contribute to the geometric spread that makes GPS positioning accurate. When buildings block them, your satellite geometry degrades slightly and your position fix becomes less precise.

Shopping centres — large low-rise structure, but surrounded by multi-level carparks, service towers, loading dock structures
Railway station precincts — overhead wiring, steel platform canopies, signal gantries, concentrated electrical infrastructure
Medium-density residential (4–8 storey) — from ground level the sky appears mostly open, but your aircraft at 60m is flying in the gap between building tops
At moderate sites you are never more than 100m from something that partially interrupts satellite geometry in at least one direction

Recommended Time Windows

06:30 – 09:00 — ✅ Optimal — satellite geometry at best, soft directional light, minimal foot traffic

09:00 – 11:00 — ⚠️ Acceptable — monitor closely, GPS starting to degrade

11:00 – 13:30 — 🚫 Avoid survey — GPS unreliable midday, overhead harsh light, highest failure rate

15:30 – 17:30 — ✅ Good secondary — GPS recovers in afternoon, directional light angle

After sunset — 🚫 Do Not Fly

Primary Failure Mode

Overconfidence. Everything looks fine at power-on. The lock is good. The numbers look acceptable. The problem surfaces 48 hours later when the processed dataset shows drift errors or misaligned patches that are difficult to correct in post-processing.

Knowledge Check

1. What is the defining characteristic of a moderate interference environment?

2. Why is RTK instability at moderate sites harder to manage than a clean RTK failure?

3. What is the best solution for RTK instability at a moderate interference site?

4. What time window should you avoid for survey or photogrammetry work at a moderate site?